1. Field of the Invention
The present invention relates to a thin-layer component composed of a thin layer of platinum or the like formed on amorphous silicon dioxide. More particularly, the invention relates to a thin-layer component composed of a thin layer of (100)-oriented platinum (Pt) or the like, which is useful in fabricating pyroelectric infrared sensors having a photosensitive layer formed of lead titanate or the like that is a ferroelectric substance. The present invention further relates to a system for producing the thin-layer component.
2. Description of the Prior Art
Pyroelectric infrared sensors, the sensitivities of which do not depend on wavelengths, are capable of operating at room temperature. They show high sensitivities even in the long-wave band so as to be useful in detection of substances at around room temperature. Thus, they are used in various fields such as industrial robots, temperature control, environmental monitoring, crime prevention, infrared cameras, or pixels in VCRs.
As the material of the pyroelectric infrared sensors, there have conventionally been used perovskite type ferroelectric thin layers that will vary in the degree of spontaneous polarization with change in temperature, particularly lead titanate (PbTiO.sub.3) by virtue of its capability of being formed into thin layers. The crystal of lead titanate is of the tetragonal system and its spontaneous polarization is oriented in the direction of the c-axis. For this reason, when the c-axis is oriented perpendicular to the substrate (c-axis orientation), any change in this spontaneous polarization can be picked up as a signal at a possible maximum level. Thus, lead titanate, when functionally applied to a pyroelectric infrared sensor, can provide such a sensor having high sensitivity.
Generally, when thin layers are formed, it is important to properly select the substrate that serves as the base. The crystal of lead titanate has a lattice constant in the a-axis direction of 3.90 .ANG., and platinum (face-centered cubic crystal), which has a lattice constant of 3.92 .ANG., is used as a substrate having an approximately equal value of lattice constant to the foregoing. If a thin layer of (100)-oriented platinum is used as a substrate and lead titanate is deposited thereon by sputtering, c-axis orientation can be rather easily achieved by virtue of its lattice matching.
Normally, platinum thin layers are formed by sputtering. The sputtering is generally implemented in the following manner: a RF or DC high voltage is applied between a target and a substrate within a vacuum chamber in which a sputter gas, such as argon gas, has been introduced, so as to generate plasma between the substrate and the target. Argon ions are generated in the plasma and are thrown into accelerated collision against the target to thereby sputter away particles of the target with the result of deposition of the latter on the substrate. Thus a thin layer is formed.
When a platinum thin-layer is formed on a (100) oriented magnesium oxide layer, the (100) orientation could be preferentially given by effecting the DC magnetron sputtering technique in the presence of a slight quantity of oxygen. Therefore, an infrared sensor incorporating lead titanate as its photosensitive material has conventionally been provided, in its basic structure, by a pyroelectric infrared sensor in which a thin layer (2) of (100)-oriented Pt is formed on a (100)-oriented magnesium oxide (MgO) substrate (3) as shown in FIG. 1(A), the thin layer being assumed to be a lower electrode, and further a c-axis oriented layer (1) of lead titanate is formed on the thin layer (2).
Meanwhile, in recent years there has been an increasing demand that sensors in the whole range including pyroelectric infrared sensors be given the ability to serve additional needs in various fields such as industrial robots, temperature control, environmental monitoring, crime prevention, infrared cameras or pixels in VCRs. To meet this demand, it is necessary to integrate a sensor portion and an IC portion together. The IC portion, normally, is provided in such a structure that the surface of the silicon substrate is protected with an amorphous layer of silicon dioxide. Thus, to fabricate a laminate-type sensor in which the sensor portion and the IC portion are integrated, it is required to form a lead titanate layer on the amorphous silicon dioxide monolithically (layer-laminated type).
Hitherto, there have been proposed various arrangements for fabricating the lead titanate layer (1) on amorphous silicon dioxide: one in which, as shown in FIG. 1(B), a (100)-oriented magnesium oxide layer (3) is formed on silicon dioxide layer (4) and further thereon a platinum layer (2) is formed by sputtering, or another in which, as shown in FIG. 1(C), a platinum layer (2) is formed on silicon dioxide (4) directly by sputtering and further thereon a lead titanate layer (1) is formed. The substrate layer (5) is under the silicon dioxide layer (4).
The sensor as shown in FIG. (B) is not preferred because magnesium oxide layer (3), if interposed between the layer (2) and the silicon dioxide layer (4) as shown in FIG. 1(B), would cause thermal load, decrease of output and deterioration of sensitivity. The substrate layer (5) is under the silicon dioxide layer (4).
When a metal layer having a crystalline structure of face-centered cubic crystal such as platinum, is formed on an amorphous substrate such as silicon dioxide, the resulting thin layer tends to take a (111)-orientation where its surface free energy becomes minimum (FIG. 1(C)). If a lead titanate layer was formed on such a (111)-oriented platinum layer, it would be difficult for lead titanate to take the c-axis orientation on account of its lattice matching. As far as the conventional sputtering technique is concerned, a platinum thin layer (2) formed on a sputtered amorphous silicon dioxide would result in (111)-orientation in most cases, so that preferentially c-axis-oriented lead titanate layers and the like could not be formed on silicon monolithically.